Surface acoustic wave device and manufacturing method therefor

ABSTRACT

In a surface acoustic wave device using a Shear Horizontal type surface acoustic wave, at least one interdigital transducer (IDT) is made of a material having a larger mass-load effect than that of aluminum. The metallization ratio of the IDT and the normalization film thickness h/λ of the IDT are controlled such that ripple caused by a transversal mode wave is about 0.5 dB or less, where “h” indicates the film thickness of the electrodes and “λ” indicates the wavelength of a surface acoustic wave.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to surface acoustic wave devices,such as surface acoustic wave resonators and surface acoustic wavefilters, and manufacturing methods therefor, and more particularly, to asurface acoustic wave device using a Shear Horizontal type (“SH-type”)surface acoustic wave and having a structure for reducing a transversalmode spurious ripple, and a manufacturing method therefor.

[0003] 2. Description of the Related Art

[0004] In surface acoustic wave devices, aluminum or alloys includingaluminum as a main component have conventionally been widely used as theelectrode material of an interdigital transducer (IDT). At least one IDTis disposed on a piezoelectric substrate and reflectors or reflectiveend surfaces are disposed at both sides of the area where the IDT islocated so as to define a resonator or a longitudinally coupledresonator filter.

[0005] In such a surface acoustic wave device, it may be possible thatthe IDT functions as a waveguide to generate a transversal mode wave,and ripples caused by the transversal mode wave are generated in a passband. To reduce the ripples caused by the transversal mode wave, variousmethods have been attempted. Those methods include a method for reducingthe intersection width of IDTs and a weighting method.

[0006] A surface acoustic wave device has also been proposed in JapaneseUnexamined patent Application publication No. Hei-11-298290, in which aquartz substrate is used, an IDT made from a metal or an alloy havingtantalum (Ta), which has a larger mass than aluminum (Al), as a maincomponent is disposed on the quartz substrate, and an SH-type surfaceacoustic wave is used. Since the IDT is made from a metal or an alloyhaving tantalum, which has a large mass, as a main component, the numberof the pairs of the electrode fingers of the IDT is as small as 10 to20, and thereby the surface acoustic wave device is made compact.

[0007] When an electrode material having a large mass-load effect, suchas a material having Ta as a main component, is used, the sonic speedobtained at the area where an IDT is located becomes much lower than thesonic speed obtained around the area. Therefore, a waveguide effect isvery large at the IDT portion.

[0008] Consequently, when a longitudinally coupled resonator filter isproduced, ripples caused by a transversal mode wave become complicatedand very large, as indicated by arrows X in FIG. 13.

[0009] As described above, as methods for removing ripples caused by atransversal mode wave from the pass band of a filter or from thevicinity of a resonant point of a resonator, a method A in which anintersection width is made small and the frequency distance between abasic-mode wave and a transversal mode wave is made large, and a methodB in which the intersection width of an IDT is weighted with a cos²function to eliminate the transversal mode wave have been conventionallyattempted.

[0010] In the method A, it is necessary to set the intersection width to10λ or less, where λ is the wavelength of a surface acoustic wave. Whena quartz substrate and an IDT having 10 to 20 pairs of electrode fingersare used to provide a surface acoustic wave device, the input and outputimpedance exceeds 2 kΩ and is very high, so that the surface acousticwave device cannot be used for actual products. Therefore, it isnecessary to increase the number of the pairs of electrode fingers toreduce the impedance.

[0011] More specifically, whereas the surface acoustic wave devicedisclosed in the above-described publication uses tantalum, which has alarge mass, as a main component to form electrodes and allows the numberof pairs in IDTs to be reduced, when the method for reducing theintersection width is used, the number of the pairs of electrode fingersneeds to be increased to reduce the input and output impedance.Therefore, the surface acoustic wave device cannot be made compact.

[0012] In the method B, weighting itself increases a loss of the surfaceacoustic wave device. In addition, since weighting reduces the area ofan intersection-width portion, the impedance of the surface acousticwave device becomes very high in the same way as in the method A.Therefore, to reduce the impedance, the intersection width needs to betwice as large as the required length. As a result, the surface acousticwave device cannot be made compact.

[0013] In other words, when either the method A or the method B is used,if ripples caused by a transversal mode wave are to be reduced, theadvantage of the surface acoustic wave device in reducing the size ofthe device as disclosed in the above-described publication is preventedfrom being achieved.

SUMMARY OF THE INVENTION

[0014] In order to overcome the problems described above, preferredembodiments of the present invention provide a surface acoustic wavedevice which has electrodes made from a material having a largermass-load effect than aluminum, which is made to be very compact, whichhas a structure that minimizes and eliminates ripples caused by atransversal mode wave, and which uses an SH-type surface acoustic wave,and a manufacturing method therefor.

[0015] According to a preferred embodiment of the present invention, asurface acoustic wave device using an SH-type surface acoustic waveincludes a quartz substrate, and at least one interdigital transducerdisposed on the quartz substrate and made from electrodes having alarger mass-load effect than that of aluminum, wherein the metallizationratio “d” and the normalized film thickness h/λ of the interdigitaltransducer are within a range such that a ripple caused by a transversalmode wave is about 0.5 dB or less, where “λ” is the wavelength of thesurface acoustic wave and “h” indicates the film thickness of theelectrodes of the at least one interdigital transducer.

[0016] In the surface acoustic wave device, the metallization ratio “d”and the normalized film thickness h/λ of the interdigital transducer arepreferably controlled so as to be within specific ranges such that theripple caused by the transversal mode wave is about 0.5 dB or less.Therefore, even in a case in which an IDT made of electrodes having alarger mass-load effect than that of aluminum is used, and the number ofthe pairs of electrode fingers is reduced to make the device verycompact, the ripple caused by the transversal mode wave is effectivelysuppressed and eliminated. Consequently, a compact surface acoustic wavedevice using an SH-type surface acoustic wave and having excellentfrequency characteristics is provided.

[0017] In the surface acoustic wave device, the interdigital transducermay include at least one electrode layer made from a metal having alarger mass than that of aluminum.

[0018] In the surface acoustic wave device, the interdigital transducermay be made from a single metal having a larger mass than that ofaluminum.

[0019] The above-described advantages are also achieved in anotherpreferred embodiment of the present invention which provides a surfaceacoustic wave device using an SH-type surface acoustic wave, including aquartz substrate, and at least one interdigital transducer disposed onthe quartz substrate and made from tantalum, wherein the normalized filmthickness h/λ of the interdigital transducer is within a range fromabout 0.6d+1.65 to about 0.6d+1.81, where “d” indicates themetallization ratio of the interdigital transducer, “λ” indicates thewavelength of the surface acoustic wave, and “h” indicates the filmthickness of the electrodes of the at least one interdigital transducer.

[0020] In the surface acoustic wave device according to this preferredembodiment, since at least one interdigital transducer made fromtantalum is disposed on the quartz substrate, and the normalized filmthickness h/λ of the interdigital transducer falls in a range from about0.6d+1.65 to about 0.6d+1.81, where “d” indicates the metallizationratio, ripples caused by a transversal mode wave are effectivelysuppressed. Therefore, even in a case in which at least one IDT madefrom tantalum is formed, and the number of the pairs of the electrodefingers in the IDT is reduced to make the device very compact, theripple caused by the transversal mode wave is effectively suppressed andeliminated. Consequently, a compact surface acoustic wave device usingan SH-type surface acoustic wave and having excellent frequencycharacteristics is provided.

[0021] The above-described advantages are also achieved in still anotherpreferred embodiment of the present invention that provides a surfaceacoustic wave device using an SH-type surface acoustic wave, including aquartz substrate, and at least one interdigital transducer disposed onthe quartz substrate and made from tungsten, wherein the normalized filmthickness h/λ of the interdigital transducer is within a range fromabout 0.6d+0.85 to about 0.6d+1.30, where “d” indicates themetallization ratio of the interdigital transducer, “λ” indicates thewavelength of the surface acoustic wave, and “h” indicates the filmthickness of the electrodes of the interdigital transducer.

[0022] In the surface acoustic wave device, since at least oneinterdigital transducer made from tungsten is disposed on the quartzsubstrate, and the normalized film thickness h/λ of the interdigitaltransducer is within a range from about 0.6d+0.85 to about 0.6d+1.30,ripples caused by a transversal mode wave are minimized and eliminatedeven in a case in which the number of the pairs of the electrode fingersin the IDT is reduced to make the device very compact. Consequently, acompact surface acoustic wave device using an SH-type surface acousticwave and having excellent frequency characteristics is provided.

[0023] According to the surface acoustic wave device having theabove-described unique structure, the transversal mode ripple isminimized to about 1.5 dB or less.

[0024] In the surface acoustic wave device according to variouspreferred embodiments of the present invention, the normalized filmthickness h/λ may fall in a range from about 0.6d+1.00 to about0.6d+1.23. In this case, the transversal mode ripple is suppressed toabout 0.5 dB or less.

[0025] In the surface acoustic wave devices described above, a pluralityof the interdigital transducers may be provided to constitute alongitudinally coupled resonator filter. In this case, a compactlongitudinally coupled resonator filter having excellent frequencycharacteristics is provided. In the surface acoustic wave devicesdescribed above, longitudinally coupled resonator filters may beconnected in a cascade arrangement in at least two stages.

[0026] In the surface acoustic wave devices described above, theinterdigital transducer may be disposed on the quartz substrate toconstitute a one-port surface acoustic wave resonator. In this case, acompact one-port surface acoustic wave resonator having excellentfrequency characteristics is provided.

[0027] The surface acoustic wave devices described above may beconfigured such that a plurality of the interdigital transducers isdisposed on the quartz substrate, wherein each of the interdigitaltransducers constitutes a one-port surface acoustic wave resonator, andthe plurality of the interdigital transducers are connected toconstitute a ladder-type filter on the quartz substrate.

[0028] The surface acoustic wave devices described above may also beconfigured such that a plurality of the interdigital transducers aredisposed on the quartz substrate, wherein each of the interdigitaltransducers constitutes a one-port surface acoustic wave resonator, andthe plurality of the interdigital transducers are connected toconstitute a lattice-type filter on the quartz substrate.

[0029] In the above two cases, compact ladder-type and lattice-typefilters having excellent frequency characteristics are provided.

[0030] The surface acoustic wave devices described above are widely usedfor surface-acoustic resonators and surface acoustic wave filters. Theforegoing advantages may also be achieved through the provision of acommunication device using one of the surface acoustic wave devicesaccording to preferred embodiments of the present invention describedabove.

[0031] The above-described advantages are achieved in yet anotherpreferred embodiment of the present invention through the provision of amanufacturing method for a surface acoustic wave device using an SH-typesurface acoustic wave, including the steps of preparing a quartzsubstrate, forming a metal film having a larger mass-load effect thanthat of aluminum on the quartz substrate, and patterning the metal filmby reactive ion etching or by a lift-off process such that themetallization ratio “d” and the normalized film thickness h/λ of theinterdigital transducer, which reduce a spurious transversal mode rippleto about 1.5 dB or less are satisfied, to form at least one interdigitaltransducer, where “d” indicates the metallization ratio of theinterdigital transducer, “λ” indicates the wavelength of a surfaceacoustic wave, and “h” indicates the film thickness of the interdigitaltransducer.

[0032] In the manufacturing method for a surface acoustic wave deviceusing an SH-type surface acoustic wave according to a preferredembodiment of the present invention, a metal film having a largermass-load effect than aluminum is disposed on the quartz substrate, andpatterning is applied to the metal film by reactive ion etching or by alift-off process such that the metallization ratio “d” and thenormalized film thickness h/λ which decrease a transversal mode rippleto about 1.0 dB or less are satisfied, to form at least one interdigitaltransducer. Therefore, even when the pair of the electrode fingers isreduced to make the device compact, the transversal mode ripple issuppressed in the device. In addition, since patterning is performed bythe reactive ion etching or the lift-off process, an IDT satisfying theforegoing normalized film thickness h/λ is always formed.

[0033] The manufacturing method for a surface acoustic wave device usingan SH-type surface acoustic wave, according to a preferred embodimentdescribed above, may be configured such that the metal film is made fromtantalum, and patterning is performed by the reactive ion etching or bythe lift-off process such that the normalized film thickness h/λ iswithin a range from about 0.6d+1.50 to about 0.65d +1.87, preferably ina range from about 0.6d+1.65 to about 0.6d+1.81, to form at least oneinterdigital transducer.

[0034] The manufacturing method for a surface acoustic wave device usingan SH-type surface acoustic wave according to the preferred embodimentdescribed above may be configured such that the metal film is made fromtungsten, and patterning is performed by the reactive ion etching or bythe lift-off process such that the normalized film thickness h/λ fallsin a range from about 0.6d+0.85 to about 0.6d+1.30, preferably in arange from about 0.6d+1.00 to about 0.6d+1.23, to form at least oneinterdigital transducer.

[0035] Other features, elements, characteristics and advantages of thepresent invention will become more apparent from the detaileddescription of preferred embodiments below with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is an outline plan view showing a surface acoustic wavedevice according to a preferred embodiment of the present invention.

[0037]FIG. 2A and FIG. 2B show the attenuation-frequency characteristicsof a surface acoustic wave device, obtained when a metallization ratio“d” is about 0.75 and a normalized film thickness h/λ is about 1.8% andabout 2.0%, respectively.

[0038]FIG. 3A and FIG. 3B show the attenuation-frequency characteristicsof a surface acoustic wave device, obtained when the metallization ratio“d” is about 0.75 and the normalized film thickness h/λ is about 2.2%and about 2.4%, respectively.

[0039]FIG. 4 shows the relationship between a normalized film thicknessh/λ and an anisotropic index obtained in a structure in which aninterdigital transducer (IDT) made from tantalum (Ta) is disposed on aquartz substrate.

[0040]FIG. 5 shows a range in which ripples caused by a transversal modewave are eliminated according to preferred embodiments of the presentinvention, and shows the relationship between the metallization ratioand the normalized film thickness h/λ.

[0041]FIG. 6A shows attenuation-frequency characteristics obtained whenthe metallization ratio “d” is about 0.75, the normalized film thicknessh/λ is about 2.15%, and the intersection width of electrode fingers isapproximately 10λ, 25λ, and 40λ;

[0042]FIG. 6B shows attenuation-frequency characteristics obtained whenthe metallization ratio “d” is about 0.75, the normalized film thicknessh/λ is about 2.15%, and the intersection width of electrode fingers isapproximately 60λ, 85λ, and 100λ.

[0043]FIG. 7 shows the relationship between a normalized film thicknessh/λ and an anisotropic index obtained in a structure in which an IDTmade from tungsten (W) is disposed on a quartz substrate.

[0044]FIG. 8 shows a range in which ripples caused by a transversal modewave are eliminated according to preferred embodiments of the presentinvention, and shows the relationship between the metallization ratioand the normalized film thickness h/λ.

[0045]FIG. 9 is an outline plan view showing a modification of thesurface acoustic wave device according to a preferred embodiment of thepresent invention.

[0046]FIG. 10 is a circuit diagram showing a ladder-type circuitdefining a filter circuit including a surface acoustic wave deviceaccording to a preferred embodiment of the present invention.

[0047]FIG. 11 is an outline block diagram showing a transmitter andreceiver in which a surface acoustic wave device according to apreferred embodiment of the present invention is used.

[0048]FIG. 12 is an outline block diagram showing another transmitterand receiver in which a surface acoustic wave device according to apreferred embodiment of the present invention is used.

[0049]FIG. 13 is a view showing an attenuation-frequency characteristicused for describing a problem with a conventional surface acoustic wavedevice.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0050] Preferred embodiments of the present invention will be describedbelow by referring to the drawings.

[0051]FIG. 1 is an outline plan view of a longitudinally coupledresonator-type filter defining as a surface acoustic wave deviceaccording to a first preferred embodiment of the present invention.

[0052] A longitudinally coupled resonator-type filter 11 preferablyincludes a substantially rectangular plate-shaped quartz substrate 12.Interdigital transducers (IDTs) 13 and 14 are disposed on the quartzsubstrate 12. The IDT 13 has a pair of comb electrodes 13 a and 13 b,the IDT 14 has a pair of comb electrodes 14 a and 14 b, and theelectrode fingers of the pair of comb electrodes are disposedalternately. The electrode fingers of the comb electrodes 13 a, 13 b, 14a, and 14 b extend in a direction that is substantially perpendicular toa surface acoustic wave propagation direction.

[0053] Therefore, the IDTs 13 and 14 are disposed in the surfaceacoustic wave propagation direction. At both sides of the portion wherethe IDTs 13 and 14 are provided, in the surface acoustic wavepropagation direction, grating reflectors 15 and 16 are preferablydisposed. The reflectors 15 and 16 have a structure in which a pluralityof electrode fingers are short-circuited at both ends, and the electrodefingers of the reflectors 15 and 16 extend in the direction that issubstantially perpendicular to the surface acoustic wave propagationdirection.

[0054] In the longitudinally coupled resonator-type surface acousticwave filter 11, the IDTs 13 and 14 and the reflectors 15 and 16 arepreferably made from tantalum (Ta), which is an electrode materialhaving a larger mass than that of aluminum (Al). In the IDTs 13 and 14,the film thickness h/λ of the IDTs 13 and 14, normalized by thewavelength of the surface acoustic wave is preferably about 0.6d+1.65 toabout 0.6d+1.81, where “d” indicates metallization ratio, “h” indicatesthe film thickness of an electrode, and “λ” indicates the wavelength ofthe surface acoustic wave.

[0055] The metallization ratio “d” refers to the ratio of the width ofan electrode finger to the sum of the width of the gap between electrodefingers in the surface acoustic wave propagation direction and the widthof the electrode finger in the surface acoustic wave propagationdirection.

[0056] In the present preferred embodiment, since the IDTs 13 and 14 aremade from Ta, which has a large mass, the number of the pairs of theelectrode fingers of the IDTs 13 and 14 can be made small, such as about19 or less, and thereby the device is made very compact.

[0057] The inventors of the present invention discovered that even whenthe IDTs 13 and 14 are made from a large-mass electrode material havingTa and the number of the pairs of the electrode fingers of the IDTs aremade small, if the normalized film thickness h/λ and the metallizationratio are within the foregoing ranges, ripples caused by a transversalmode wave are effectively minimized and eliminated to achieve theadvantages of preferred embodiments of the present invention.

[0058] Specific experimental examples of preferred embodiments will beused to describe the present invention.

[0059] A quartz substrate having a quartz orientation indicated by Eulerangles (0°, 127°, 90°) was used as the quartz substrate 12, and the IDTs13 and 14 and the reflectors 15 and 16 were formed by using Ta as anelectrode material on the quartz substrate 12. The number of the pairsof the electrode fingers of the IDTs 13 and 14 was set to 13, and thenumber of the electrode fingers of the reflectors 15 and 16 was set to10. Various longitudinally coupled resonator-type surface acoustic wavefilters 11 using -an SH wave were made so as to have differentnormalized film thickness h/λ in a range from about 0.017 to about 0.025while the metallization ratio “d” of the IDTs 13 and 14 was set within arange of about 0.5 to about 0.90. FIG. 2 and FIG. 3 show theattenuation-frequency characteristic of some of the surface acousticwave devices made as described above. FIG. 2A shows a characteristicobtained when the metallization ratio “d” was about 0.75 and h/λ wasabout 0.018. FIG. 2B shows a characteristic obtained when themetallization ratio “d” was about 0.75 and h/λ was about 0.02. FIG. 3Ashows a characteristic obtained when the metallization ratio “d” wasabout 0.75 and h/λ was about 0.022. FIG. 3B shows a characteristicobtained when the metallization ratio “d” was about 0.75 and h/λ wasabout 0.024.

[0060] In FIG. 2A to FIG. 3B, dotted lines indicate characteristics withan expanded attenuation scale shown in the vertical axis at the right.

[0061] In the characteristic shown in FIG. 2A, it is seen that a rippleindicated by an arrow Al was generated at the center of a pass band, anda number Y1 to Y3 of ripples appeared at the lower-frequency side of thepass band. Also in the characteristic shown in FIG. 2B, it is seen thatlarge ripples appeared in the pass band as indicated by an arrow A2, andripples were generated at the lower-frequency side of the pass band asindicated by arrows Y4 to Y6.

[0062] In the characteristic shown in FIG. 3B, it is seen that largeripples indicated by arrows A3 and A4 were generated in the pass band,and ripples Y7 and Y8 appeared at the higher-frequency side of the passband.

[0063] In contrast, in the characteristic shown in FIG. 3A, very small,negligible ripples appeared in the pass band, and very small ripplesappeared in the vicinity of the pass band even in the lower-frequencyside and the higher-frequency side of the pass band.

[0064] Therefore, even when the metallization ratio “d” and thenormalized film thickness h/λ are adjusted to reduce the number of thepairs of electrode fingers to make the device compact, ripples caused bya transversal mode wave is still effectively minimized.

[0065] In other words, preferred embodiments of the present inventionprovide advantages in that even when IDTs are made of a metallicmaterial, such as tantalum, having a larger mass than that of aluminumin order to reduce the number of electrode fingers to make the devicecompact, the metallization ratio “d” and the normalized film thicknessh/λ of the IDTs are selected so as to minimize ripples caused by atransversal mode wave.

[0066] As described above, when IDTs are made of a metal having a largemass, the sonic speed obtained in the IDTs 13 and 14 becomes much slowerthan that obtained around them, and a waveguide effect becomes strong.Therefore, as described with the surface acoustic wave device disclosedin the foregoing publication, ripples caused by a transversal mode wavebecome large. It is clear from FIG. 2A to FIG. 3B, however, that ripplescaused by a transversal mode wave does occur only to a very small degreeat a film thickness “h” falling in the specific desired range. When theactual film thickness is thinner than a film thickness within thespecific desired range, ripples caused by a transversal mode wave appearat the lower-frequency side of the pass band as shown in FIG. 2A andFIG. 2B. When the actual film thickness is thicker than a film thicknesswithin the specific desired range, ripples caused by a transversal modewave appear at the higher-frequency side of the pass band as shown inFIG. 3B.

[0067] In surface acoustic wave devices having IDTs made from a materialhaving aluminum as a main component, the above-described phenomenon doesnot occur in which the frequency where ripples occur shifts from thelower-frequency side to the higher-frequency side according to the filmthickness. When the IDTs 13 and 14 are made of a metallic material, suchas tantalum, having a larger mass than that of aluminum, theabove-described ripple-frequency-shift phenomenon occurs. The followingcause thereof can be expected.

[0068] There is an analysis method using a waveguide model to calculatethe frequency where a transversal mode wave is generated. According tothis analysis method, the cause will be described below.

[0069] The direction that is substantially perpendicular to theelectrode fingers of the IDTs 13 and 14 is set to a reference 0 (rad),and the sonic speed of a surface acoustic wave propagating in adirection shifted from the reference 0 by an angle θ (rad) is set toV_(saw)(θ) . Then, V_(saw)(θ) is approximated in terms of θ by aquadratic function V_(saw)(θ)=Vo{1 +(γ/2)θ² }, where γ is called theanisotropic index of the sonic speed in a substrate and described invarious documents. In an ST-cut quartz substrate, for example, γ is0.378.

[0070] When the frequency where a transversal mode wave is generated iscalculated by the waveguide model with γ being introduced, if γ islarger than −1, the transversal mode wave occurs at the higher-frequencyside than the basic-mode wave. If γ is smaller than −1, the transversalmode wave occurs at the lower-frequency side than the basic-mode wave.

[0071] Therefore, in the ST-cut quartz substrate, the transversal modewave occurs at the higher-frequency side than the basic-mode wave. It isactually known that the transversal mode occurs at the higher-frequencyside than the basic mode in surface acoustic wave devices havingelectrodes made from aluminum and using a Rayleigh wave.

[0072] The anisotropic index γ of the sonic speed in a substrate needsto be obtained with the mass of electrodes being taken intoconsideration when electrodes are disposed on the substrate. Therefore,γx indicates the anisotropic index of the sonic speed, in which the massof electrodes is taken into consideration, in the following description.

[0073] The inventors of the present invention discovered based on theabove-described fact that the following can be considered. Specifically,when an electrode material having aluminum as a main component, which iswidely used for currently available surface acoustic wave devices, isused, a slight change in γx caused by the effects of the film thicknessof the electrodes and the metallization ratio occurs and γx does notshift largely from γ of the substrate itself. When an electrode materialhaving a large mass, such as tantalum, is used, however, the mass-loadeffect of the electrodes, namely, the film thickness of the electrodes,changes γx greatly. It can be expected that when the film thickness ofthe electrodes is small, γx<−1 and the transversal mode wave isgenerated at the lower-frequency side of the pass band, and when thefilm thickness of the electrodes is large, γx gradually increases to beγx>−1 and the transversal mode wave is generated at the higher-frequencyside of the pass band. It can also be expected that since it isdifficult for the transversal mode wave to occur in the vicinity ofγx=−1, which is between γx<1 and γx>−1, ripples caused by thetransversal mode wave can be reduced or can be completely removed asshown in FIG. 3A.

[0074] To prove the foregoing expectation, γx was estimated by thefinite-element method. More specifically, the metallization ratio “d”was set to about 0.75, the normalized film thickness h/λ was changed,and the anisotropic index γx was observed. FIG. 4 shows the result.

[0075] It is clear from FIG. 4 that γx shifted from the area where γx<−1to the area where γx>−1 at a boundary disposed in the vicinity ofh/λ=2.2%.

[0076] With the foregoing experimental result being taken intoconsideration, a range where it is considered that the ripples caused bythe transversal mode wave are substantially eliminated, that is, a rangewhere the ripples are about 0.5 dB or less, was obtained. As a result,it was determined that when the metallization ratio was about 0.75, thenormalized film thickness h/λ should be about 2.10% to about 2.25%, andγx should be about −1.10 to about −0.96.

[0077] The metallization ratio “d” was changed in the surface acousticwave device 11 to have values, including the above-described value, thefinite-element method was used in the same way, and areas where theripple could be reduced to about 1.5 dB or less or about 0.5 dB or lesswere obtained. FIG. 5 shows the result.

[0078] It is clear from FIG. 5 that the normalized film thickness shouldbe within a range of about 0.6d+1.50 to about 0.6d+1.87 to make theripples in the band caused by the transversal mode wave to be about 1.5dB or less, and to fall within a range of about 0.6d+1.65 to about0.6d+1.81, both inclusive, to make the ripples in the band caused by thetransversal mode wave to be about 0.5 dB or less. As described above, itis understood that the metallization ratio “d” and the normalized filmthickness h/λ are adjusted in the longitudinally coupled resonator-typesurface acoustic wave filter 11 to effectively suppress the ripplescaused by the transversal mode wave without weighting the intersectionwidth. In other words, the ripples caused by the transversal mode wavecan be effectively minimized without preventing the longitudinallycoupled resonator-type surface acoustic wave filter 11 from being madecompact.

[0079] The inventors of the present invention also checked the degree ofan effect caused by a change in the intersection width of the electrodefingers of the IDTs on the ripples caused by the transversal mode wave.FIG. 6A and FIG. 6B show attenuation-frequency characteristics obtainedin the longitudinally coupled resonator-type surface acoustic wavefilter 11 when the IDTs 13 and 14 were made of Ta, the metallizationratio “d” was set to about 0.75, and h/λ was set to about 2.15% in thesame way as in the above-described experiment, and the intersectionwidth of the electrode fingers was changed to approximate values of 10λ,25λ, 40λ, 60λ, 80λ, and 100λ. In FIG. 6A and FIG. 6B, dotted linesindicate main portions of the attenuation-frequency characteristics withan expanded scale shown at the right.

[0080] It is clear from FIG. 6A and FIG. 6B that even when theintersection width was largely changed, the ripples caused by thetransversal mode wave did not occur.

[0081] The IDTs were made from Ta in the experiments shown in FIG. 4 andFIG. 5. In the present invention, however, the electrode material usedfor the IDTs is not limited to Ta.

[0082] The inventors of the present invention also made the IDTs 13 and14 with tungsten (W) instead of Ta, checked the change of theanisotropic index γx while the normalized film thickness was changed,and obtained the range of the normalized film thickness, where theripples in the band can be reduced to about 1.5 dB or less or about 0.5dB or less while the metallization ratio “d” was changed, in the sameway as in the experiments shown in FIG. 4 and FIG. 5. FIG. 7 and FIG. 8show the results.

[0083] It is clear from FIG. 7 that when the metallization ratio “d” wasabout 0.75, the normalized film thickness h/λ should be about 1.3% toabout 1.75% to make the ripples be about 1.5 dB or less, and should beabout 1.45% to about 1.68% to make the ripples be about 0.5 dB or less.It is clear from FIG. 8 that when the metallization ratio “d” waschanged, the normalized film thickness should be within a range of about0.6d+0.85 to about 0.6d+1.30 to make the ripples in the band caused bythe transversal mode wave to be about 1.5 dB or less, and should bewithin a range of about 0.6d+1.00 to about 0.6d+1.23 to make the ripplesin the band caused by the transversal mode wave to be about 0.5 dB orless.

[0084] In the foregoing preferred embodiments of the present invention,a one-stage longitudinally coupled resonator-type surface acoustic wavefilter has been described. Two longitudinally coupled resonator-typesurface acoustic wave filters may be connected in a cascade arrangementto define a surface acoustic wave device 21 shown in FIG. 9. In thiscase, a first-stage longitudinally coupled resonator-type surfaceacoustic wave filter includes IDTs 23 and 24 and reflectors 25 and 26,and a second-stage longitudinally coupled resonator-type surfaceacoustic wave filter includes IDTs 27 and 28 and reflectors 29 and 30.The first-stage and second-stage longitudinally coupled resonator-typesurface acoustic wave filters preferably have the same structure as thelongitudinally coupled resonator-type surface acoustic wave filter shownin FIG. 1. Among the comb electrodes 23 a and 23 b of the IDT 23 of thefirst longitudinally coupled resonator-type surface acoustic wavefilter, one comb electrode 23 b is electrically connected to one combelectrode 28 a among the comb electrodes 28 a and 28 b of the IDT 28 ofthe second-stage longitudinally coupled resonator-type surface acousticwave filter.

[0085] Also in the surface acoustic wave device 21, when the IDTs 23,24, 27, and 28 are constructed in the same way as in the above-describedpreferred embodiment, ripples caused by a transversal mode wave areeffectively minimized and eliminated.

[0086] A surface acoustic wave device according to the present inventionis not limited to the above-described longitudinally coupledresonator-type surface acoustic wave filters. Specifically, also in aone-port surface acoustic wave resonator 31 shown in FIG. 10, when anIDT 33 disposed on a quartz substrate 32 is constructed in the same wayas the IDTs 13 and 14 described in the foregoing preferred embodiment,ripples caused by a transversal mode wave are suppressed. In FIG. 10,there are also shown comb electrodes 33 a and 33 b and reflectors 34 and35.

[0087] In addition, the present invention can also be applied to varioussurface acoustic wave filters in which a plurality of the one-portsurface acoustic wave resonators is disposed and the plurality of thesurface acoustic wave resonator is electrically connected to definefilter circuits. As shown in FIG. 11, for example, a plurality ofone-port SAW resonators may be connected on a quartz substrate so as todefine a ladder-type filter having a plurality of series resonators S1to S3 and a plurality of parallel resonators P1 to P4. In the same way,a plurality of one-port surface acoustic wave resonators may beconnected to define a lattice-type circuit.

[0088] In the foregoing preferred embodiment, the IDTs 13 and 14 arepreferably made of Ta. In the present invention, an IDT may be made froma metal having a larger mass than that of aluminum. An IDT is notnecessarily made from a single metal material. Unless the entire IDT hasa smaller mass than an IDT made from aluminum, it may have a structurein which a plurality of electrode layers are laminated to define theIDT. In this case, at least one electrode layer should be made from ametal or an alloy having a larger mass than that of aluminum.

[0089] The present invention can also be applied to various surfaceacoustic wave devices, such as surface acoustic wave resonators andsurface acoustic wave filters. When the present invention is applied toa surface acoustic wave filter, for example, it can function as a bandfilter of a mobile transmitter and receiver.

[0090] In FIG. 12, an antenna 161 is connected to a duplexer 162.Between the duplexer 162 and a receiving-side mixer 163, a surfaceacoustic wave filter 164 and an amplifier 165 are connected, whichconstitute an RF stage. An IFstage surface acoustic wave filter 169 isconnected to the mixer 163. Between the duplexer 162 and atransmitting-side mixer 166, an amplifier 167 and a surface acousticwave filter 168 are connected, which constitute an RF stage.

[0091] A surface acoustic wave device according to various preferredembodiments of the present invention can be successfully used as thesurface acoustic wave filter 169 in the above transmitting and receivingmachine 160.

[0092] In a surface acoustic wave device of a preferred embodiment ofthe present invention, at least one IDT including an electrode structurehaving a larger mass effect than that of Al is disposed on a quartzsubstrate, and the IDT metallization ratio “d” and the normalized filmthickness h/λ are within specific ranges such that ripple caused by atransversal mode wave is about 0.5 dB or less. In this case, to form anIDT that satisfies the desired normalized film thickness h/λ, a methodis suitably used in which a metal film is formed on a quartz substrate,and reactive ion etching or a lift-off process is used to performpatterning to form at least one IDT. To form an IDT made from a materialhaving aluminum as a main component, patterning is widely performed bywet etching conventionally. Wet etching is not suited to very finemachining, and it is impossible to use wet etching to form an IDT havinga line width satisfying the above-described specific desired normalizedfilm thickness h/λ. Electrode fingers having a line width satisfying thedesired normalized film thickness h/λ can be formed with high precisionby using a patterning method including reactive ion etching or alift-off process.

[0093] In a surface acoustic wave device according to a preferredembodiment of the present invention, when it is necessary to polish aquartz substrate to adjust a frequency, or when an electrode layer isformed under an electrode layer made from a metal, such as Ta, having alarge mass, the film thickness of an IDT should be set to an equivalentvalue to the above-described desired normalized film thickness h/λ, withan effect caused by the polishing of the quartz substrate on a mass loadcaused by the electrode layer having a larger mass than that ofaluminum, or the mass-load operation of the electrode layer disposedunder being taken into account comprehensively. With that setting,ripples caused by a transversal mode wave are effectively minimized andeliminated in the same way as in the preferred embodiments describedabove.

[0094] While preferred embodiments of the invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the invention. The scope of the invention, therefore, is to bedetermined solely by the following claims.

What is claimed is:
 1. A surface acoustic wave device using a ShearHorizontal type surface acoustic wave, comprising: a quartz substrate;and at least one interdigital transducer disposed on the quartzsubstrate and including electrodes having a larger mass-load effect thanthat of aluminum; wherein a metallization ratio “d” and a normalizedfilm thickness h/λ of the at least one interdigital transducer arewithin specific ranges such that a ripple caused by a transversal modewave is about 0.5 dB or less, where “λ” is the wavelength of the surfaceacoustic wave and “h” is the film thickness of the electrodes of the atleast one interdigital transducer.
 2. A surface acoustic wave deviceaccording to claim 1, wherein the at least one interdigital transducerincludes at least one electrode layer made from a metal having a largermass than that of aluminum.
 3. A surface acoustic wave device accordingto claim 1, wherein the at least one interdigital transducer is madefrom a single metal having a larger mass than that of aluminum.
 4. Asurface acoustic wave device according to claim 1, further comprising aplurality of the interdigital transducers arranged to constitute alongitudinally coupled resonator filter.
 5. A surface acoustic wavedevice according to claim 4, further comprising a plurality of thelongitudinally coupled resonator filters, which are connected in acascade arrangement in at least two stages.
 6. A surface acoustic wavedevice according to claim 1, wherein the at least one interdigitaltransducer is arranged on the quartz substrate to constitute a one-portsurface acoustic wave resonator.
 7. A surface acoustic wave deviceaccording to claim 1, wherein a plurality of the interdigitaltransducers are disposed on the quartz substrate; each of the pluralityof interdigital transducers constitutes a one-port surface acoustic waveresonator; and the plurality of the interdigital transducers areconnected to constitute a ladder-type filter on the quartz substrate. 8.A surface acoustic wave device according to claim 1, wherein a pluralityof the interdigital transducers are disposed on the quartz substrate;each of the plurality of interdigital transducers constitutes a one-portsurface acoustic wave resonator; and the plurality of the interdigitaltransducers are connected to constitute a lattice-type filter on thequartz substrate.
 9. A communication device comprising a surfaceacoustic wave device according to claim
 1. 10. A surface acoustic wavedevice using a Shear Horizontal type surface acoustic wave, comprising:a quartz substrate; and at least one interdigital transducer disposed onthe quartz substrate and made from tantalum; wherein a normalized filmthickness h/λ of the at least one interdigital transducer is within arange of about 0.6d +1.65 to about 0.6d+1.81, where “d” is themetallization ratio of the interdigital transducer, “λ” is thewavelength of the surface acoustic wave, and “h” is the film thicknessof the electrodes of the at least one interdigital transducer.
 11. Asurface acoustic wave device according to claim 10, further comprising aplurality of the interdigital transducers arranged to constitute alongitudinally coupled resonator filter.
 12. A surface acoustic wavedevice according to claim 10, further comprising a plurality of thelongitudinally coupled resonator filters, which are connected in acascade arrangement in at least two stages.
 13. A surface acoustic wavedevice according to claim 10, wherein the at least one interdigitaltransducer is arranged on the quartz substrate to constitute a one-portsurface acoustic wave resonator.
 14. A surface acoustic wave deviceaccording to claim 10, wherein a plurality of the interdigitaltransducers are disposed on the quartz substrate; each of the pluralityof interdigital transducers constitutes a one-port surface acoustic waveresonator; and the plurality of the interdigital transducers areconnected to constitute a ladder-type filter on the quartz substrate.15. A surface acoustic wave device according to claim 10, wherein aplurality of the interdigital transducers are disposed on the quartzsubstrate; each of the plurality of interdigital transducers constitutesa one-port surface acoustic wave resonator; and the plurality of theinterdigital transducers are connected to constitute a lattice-typefilter on the quartz substrate.
 16. A communication device comprising asurface acoustic wave device according to claim
 10. 17. A surfaceacoustic wave device using a Shear Horizontal type surface acousticwave, comprising: a quartz substrate; and at least one interdigitaltransducer disposed on the quartz substrate and made from tungsten;wherein a normalized film thickness h/λ of the at least one interdigitaltransducer is within a range of about 0.6d +0.85 to about 0.6d+1.30,where “d” is the metallization ratio of the interdigital transducer, “λ”is the wavelength of the surface acoustic wave, and “h” is the filmthickness of the electrodes of the at least one interdigital transducer.18. A surface acoustic wave device according to claim 17, wherein thenormalized film thickness h/λ is within a range of about 0.6d+1.00 toabout 0.6d+1.23.
 19. A surface acoustic wave device according to claim17, further comprising a plurality of the interdigital transducersarranged to constitute a longitudinally coupled resonator filter.
 20. Asurface acoustic wave device according to claim 19, further comprising aplurality of the longitudinally coupled resonator filters, which areconnected in a cascade arrangement in at least two stages.
 21. A surfaceacoustic wave device according to claim 17, wherein the at least oneinterdigital transducer is arranged on the quartz substrate toconstitute a one-port surface acoustic wave resonator.
 22. A surfaceacoustic wave device according to claim 17, wherein a plurality of theinterdigital transducers are disposed on the quartz substrate; each ofthe plurality of interdigital transducers constitutes a one-port surfaceacoustic wave resonator; and the plurality of the interdigitaltransducers are connected to constitute a ladder-type filter on thequartz substrate.
 23. A surface acoustic wave device according to claim17, wherein a plurality of the interdigital transducers are disposed onthe quartz substrate; each of the plurality of interdigital transducersconstitutes a one-port surface acoustic wave resonator; and theplurality of the interdigital transducers are connected to constitute alattice-type filter on the quartz substrate.
 24. A communication devicecomprising a surface acoustic wave device according to claim
 17. 25. Amethod for manufacturing a surface acoustic wave device using a ShearHorizontal type surface acoustic wave, comprising the steps of:preparing a quartz substrate; forming a metal film having a largermass-load effect than that of aluminum on the quartz substrate; andpatterning the metal film to form at least one interdigital transducerby one of reactive ion etching and a lift-off process such that ametallization ratio “d” and a normalized film thickness h/λ of the atleast one interdigital transducer which makes a spurious transversalmode ripple to be about 1.5 dB or less are satisfied, where “d” is themetallization ratio of the interdigital transducer, “λ” is thewavelength of a surface acoustic wave, and “h” is the film thickness ofthe interdigital transducer.
 26. A method according to claim 25, whereinthe metal film is made from tantalum, and patterning is performed suchthat the normalized film thickness h/λ is within a range of about0.6d+1.50 to about 0.65d+1.87 to form the at least one interdigitaltransducer.
 27. A method according to claim 26, wherein patterning isperformed such that the normalized film thickness h/λ of the at leastone interdigital transducer is within a range of about 0.6d+1.65 toabout 0.6d+1.81.
 28. A method according to claim 25, wherein the metalfilm is made from tungsten, and patterning is performed such that thenormalized film thickness h/λ is within a range from about 0.6d+0.85 toabout 0.6d+1.30 to form the at least one interdigital transducer.
 29. Amethod according to claim 28, wherein patterning is performed such thatthe normalized film thickness h/λ of the interdigital transducer iswithin a range from about 0.6d+1.00 to about 0.6d+1.23.